Negative effective electron affinity emitters with drift fields using deep acceptor doping

ABSTRACT

An electron emitter comprising a body of gallium phosphide having a thin surface layer of cesium. The gallium phosphide is doped with a deep acceptor such as iron. Interaction between the cesium layers and the semiconductor surface results in ionization of the deep acceptor impurities in a small region near the surface. The ionization of deep acceptors at the cesiated surface results in a graded concentration of ionized impurities through the gallium phosphide layer, which establishes an internal electric field for impelling electrons toward the cesiated emitting surface.

United States Patent Simon et al.

[151 3,699,404 [4 1 Oct. 17, 1972 [54] NEGATIVE EFFECTIVE ELECTRON3,105,166 9/1963 Choyke et a1 ..313/310 AFFINITY EMITTERS WITH DRIFT3,150,282 9/ 1964 Geppert 13/346 FIELDS USING DEEP ACCEPTOR 3,422,3221/1969 Haisty ..'317/235 DOPING 3,478,213 11/1969 Simon et a1 ..250/2071 1 [72] Inventors: Ralph E. Simon, Trenton; Brown F. g ,3 X22 22 at a22 22 Williams, Princeton, both of NJ.

[73] Assignee: RCA Corporation OTHER PUBLICATIONS Feb. 24, Eigeggert,Proceedings VIC]. 54, NO. I, Jan.

[21] App]. No.: 118,491 .1. Scheer, Philips Res. Reports, 15, 584(1960).

Relaed Us Application Data I Sgigeergg; a1., SOIld State Communications,3, 189- [63] Continuation of Ser. No. 751,862, Aug. 12,

' 1968, abandoned. Primary Examiner-Martin H. Edlow Attorney--Glenn H.Bruestle [52] US. Cl. ....3l7/235 R, 317/235 N, 317/235 UA,

317/235 [57] ABSTRACT [51] Int. Cl. ..H0ll 15/00 An electron emitter comprislng .a body of gallium [58] new of gkgg 5 2 6 phosphide having athin surface layer of cesium. The

gallium phosphide is doped with a deep acceptor such 56 R fare c Citedas iron. Interaction between the cesium layers and the 1 e n essemiconductor surface results in ionization of the deep UNITED STATESPATENTS acceptor impurities in a small region near the surface,

The ionization of deep acceptors at the cesiated sur- 3,422,322 H1969Haisty .317/235 face results in a graded concentration of ionized impu3,458,782 7/1969 Buck ......317/235 rities through h gallium phosphidelayer, which 3,121,809 2/1964 Afana "307/885 establishes an internalelectric field. for impelling e1ec- 3,478,213 1 81111011 trons towardthe cesiated emitting surface.

3,387,161 6/1968 Van Laar ..313/94 2,960,659 1 H1960 Burton ..330/65 6Claims, 6 Drawing Figures f PATENTEDUBI 1'1 am sum 20F 3 IITOIII'YNEGATIVE EFFECTIVE ELECTRON AFFINITY EMITTERS WITH DRIFT FIELDS USINGDEEP ACCEPTOR DOPING This application is a continuation of Ser. No.751,862, filed 8/12/68, now abandoned.

BACKGROUND OF THE INVENTION This invention relates to electron emittersor cathodes, and more particularly to an electron emitter whichcomprises a body of semiconductor material.

A particular type of cold cathode semiconductor electron emitter hasrecently been discovered in which electron emission is obtained bymanufacturing a semiconductor structure such that the bottom of theconduction band in the semiconductor bulk lies at an energy level abovethe vacuum energy level at the emitting surface. Thus, conduction bandelectrons may drift to the emitting surface with a residual energy abovethat of the vacuum energy level, so that these electrons may be emittedfrom the surface without the necessity of supplying additional energy.Such a structure is known as a negative electron affinity emitter, andis described in the following U. S. Patent Applications:

i. Ser. No. 668,130; filed Sept. 15, 1967; Entitled: SemiconductorElectron Emitter now issued as US Pat. No. 3,478,213

ii. Ser. No. 665,511; filed Sept. 5, 1967; Entitled: Transmission TypeSecondary Emission Device with Semiconductor Dynode.

SUMMARY OF THE INVENTION An electron emitter is provided which comprisesa P type semiconductor body including a deep acceptor impurity. Anelectropositive work function reducing layer is disposed on a givensurface of the semiconductor body. The ionization energy of the deepacceptor does not exceed the difference between the energy gap of thesemiconductor body and the work function of the electropositive layer onthe semiconductor surface.

DRAWINGS FIG. 1 shows a cross-sectional view of a secondary emissiondynode according to a preferred embodiment of the invention;

FIG. 2 shows a cross-sectional view of an injection type electronemitter according to an alternative embodiment of the invention;

FIG. 3a shows a partial cross-sectional view of the active portion ofthe dynode shown in FIG. 1;

FIG. 3b shows an energy band diagram corresponding to the partialcross-sectional view of FIG. 30;

FIG. 4a shows a partial cross-sectional view of the active portion ofthe injection emitter shown in FIG. 2; and

FIG. 4b shows an energy band diagram corresponding to the partialcross-sectional view of FIG. 4a.

DETAILED DESCRIPTION A secondary emission dynode l utilizing thenegative electron affinity principle, as shown in FIG. 1, comprises analumina substrate 2 upon which is disposed a thin layer 3 comprising an(shallow) acceptor impurity material such as beryllium. A galliumphosphide P type semiconductor layer 4 is disposed on the berylliumlayer 3. A thin cesium layer 5, which may have a thickness on the orderof a few atomic diameters, is disposed on the gallium phosphide layer 4.

The gallium phosphide layer 4 is doped with a deep acceptor impuritymaterial such as iron, chromium or copper. Interaction between theelectropositive cesium layer 5 and the gallium phosphide P type layer 4results in ionization of the deep acceptors near the interface betweenthese layers.

As a result, the surface of the gallium phosphide layer 4 adjacent thecesium layer 5 behaves as highly doped (i.e., degenerate or nearlydegenerate) P type semiconductor material. The energy band structure ofthe dynode 1 is such that electrons in the conduction band of thesemiconductor material comprising the layer 4 can reach the exposedsurface 6 of the cesium layer 5 with sufficient residual energy tosurmount the potential barrier at this surface and be emitted therefrom.

The dynode 1 may be employed as a secondary emission type electronmultiplier by bombarding the exposed surface 6 of the cesium layer 5with primary electrons in a direction such as that indicated by thearrow 7. Secondary electrons which are emitted from the surface 6 may bedrawn off by a suitable electric field in the direction indicated by thearrow 8. Typically, the primary electrons may have energies in the rangeof 300 to 10,000 electron volts.

By reducing the thickness of the alumina substrate 2 to a value on theorder of 500 Angstroms, the dynode I may be employed as a transmissiontype secondary emission electron multiplier. In this structure, primaryelectrons are directed at the exposed surface of the alumina layer 2;the primary electrons penetrate the alumina layer 2 and the berylliumlayer 3 to reach the gallium phosphide layer 4, where secondaryelectrons are produced and emitted from the exposed surface 6 of thecesium layer 5. Since approximately 40 percent of the energy of theincident primary electrons is lost in passing through the alumina andberyllium layers, relatively high primary electron energies, on theorder of 3,000 to 5,000 electron volts or more, are desirable when thedynode l is operated as such a transmission type electron multiplier.

Typically, the beryllium layer 3 may have a thickness on the order of afew atomic diameters, and the gallium phosphide layer 4, which may bemonocrystalline or polycrystalline, may have a thickness on the order of0.2 to 1 micron. During fabrication of the dynode 1, heat treatmentcauses beryllium from the layer 3 to diffuse a short distance (on theorder of 10 to I00 Angstroms) into the gallium phosphide layer 4 to forma thin highly doped P type surface region.

The use of a deep acceptor impurity in the gallium phosphide layer 4results, as will hereafter be explained in detail, in an internalelectric field (drift field) within the layer 4 which impels electronstoward the cesium layer 5, thus increasing the external quantumefficiency of the dynode I.

In the structure shown in FIG. 1, electrons are introduced into theconduction band of the gallium phosphide layer 4 by bombarding thislayer with primary electrons.

Alternatively, electrons may be introduced into the conduction band ofthe gallium phosphide layer 4 by injection, utilizing a forward biasedP-N junction. Such a structure is exemplified by the injection typenegative affinity electron emitter 10, as shown in FIG. 2.

Theinjection type electron emitter 10 comprises a highly doped N typegallium phosphide substrate 11, which may be doped with a suitabledonorimpurity such as tellurium. Ohmic electrical contact to the substrate 1l is provided by means of a tin electrode 12. An insulating layer 13comprising, e.g., pyrolytically deposited silicon dioxide is depositedupon the upper surface of the gallium phosphide substrate 11. Theinsulating layer 13 has an aperture 14 exposing a portion ,of thegallium phosphide surface.

A P type gallium phosphide layer 15 is disposed on the insulating layer13 and in the aperture 15 so that the P type layer 15 forms a P-Njunction 16. with the underlying portion of the N type gallium phosphidesubstrate 11. A nickel electrode layer 17 provides ohmic electricalcontact to the P type gallium phosphide layer 15. The layer 15 includesa deep acceptor impurity such as iron, chromium or copper.

A thin cesium layer 18 is disposed on the exposed surface of the P typelayer 15, the layers 18 and 15 cooperating to form a negative affinityelectron emitting structure which operates in similar fashion to thatprovided by the layers and 4 of the dynode l. Electrons are introducedinto the conduction band of the P type gallium phosphide layer 15 byinjection from the N type substrate 1 1 across the forward biased P-Njunction 16. Injection biasing is provided for the P-N junction 16 bymeans of a battery 19 and a series resistor 20 connected between theelectrodes 12 and 17.

The injected electrons diffuse or drift through the P type layer 15 andare emitted from the exposed surface of the cesium layer 18.

The operation of the dynode 1 will be better understood by reference toFIGS. 3a and 3b.

As shown inpartial cross-sectional view in FIG. 3a, the active portionof the dynode 1 comprises the P type gallium phosphide layer 4 and thecesium surface layer 5. A thin P region 21 is provided by the diffusionof beryllium into the gallium phosphide layer 4 from the metallicberyllium layer 3. The total thickness 11 of the gallium phosphide layer4 may typically be on the order of 0.2 to 1 micron, while the thicknessa of the P re gion 21 may be on the order of to 100 Angstroms, i.e.substantially less than the total thickness of the semiconductor layer4. The thickness c of the cesium layer 6 may be on the order of l to 10Angstroms.

The energy band diagram for the active portion of the dynode 1 is shownin FIG. 3b, which is vertically aligned with FIG. 3a. The energy gapbetween the valence and conduction bands has a value E,, while theionization energy of the deep acceptor (iron, chromium or copper)impurity in the gallium phosphide layer 4 has a value E An "intrinsicline is drawn on the diagram midway between the valence and conductionbands. As is well known, portions of the semiconductor layer in whichthe Fermi level lies below the intrinsic line exhibit P typeconductivity, while portions of the layer in which the Fermi level isabove the intrinsic line exhibit N type conductivity.

By the ionization energy I5 of the deep acceptor impurity is meant theenergy which must be imparted to each impurity site to generate a holethereat.

By the term deep acceptor impurity is meant an acceptor impurity whichis not normally ionized at the operating temperature of thesemiconductor body. Since a thermal energy of kT electron volts isavailable at any particular absolute temperature T (k being theBoltzmann constant), the ionization energy of a deep acceptor impuritymust be greater than this value. We prefer to employ deep acceptorimpurities having ionization energies on the order of 4 kT or more.Since the value of kT at room temperature (300 C.) is 0.026 electronvolt, deep acceptors used in conjunction with the structure describedherein should preferably have room temperature ionization energies onthe order of at least 0.1 electron volt.

The electropositive cesium layer 5 pins the bottom of the .conductionband to the Fermi level at the emitting surface of the P type layer 4,as illustrated in FIG. 3b. The work function "I at the emitting surface6 may be defined as the difference between the vacuum energy level andthe Fermi level at this surface.

The pinning of the bottom of the conduction band at the Fermi level atthe emitting surface necessitates a sharp bending of the valence andconduction bands in the immediate vicinity of the emitting surface. Theelectropositive cesium layer 5 introduces electrons into the adjacentportion of the gallium phosphide layer 4, so that a thin N typeinversion region exists at the emitting surface. This inversion layerhas a thickness 8, and terminates at the point where the Fermi levelcrosses the intrinsic line.

In the region where the Fermi level and the deep acceptor level cross,the deep acceptors are ionized. These deep acceptors are ionized with anenergy spread of a few times kT from the Fermi level. The ionization ofthese deep acceptors contributes to the sharp band bending at theemitting surface.

Typically, for gallium phosphide containing iron as the deep acceptor,with an iron concentration on the order of 10" to 10 atoms/emf, the deepacceptors are ionized for a distance of approximately Angstroms from theinterface with the cesium layer 5.

The proportion of the deep acceptor impurities which are ionizeddecreases with distance away from the emitting surface, resulting in avarying distance between the deep acceptor level and the Fermi level.Since the valence and conduction bands are necessarily parallel to thedeep acceptor level, these bands are therefore sloped, resulting in aninternal potential gradient, i.e. an electric field oriented in adirection which impels electrons toward the emitting surface.

Electrons in the conduction band of the semiconductor layer 4 (havingbeen introduced into the conduction band by secondary or avalanchegeneration, photon generation of hole-electron pairs, injection,tunneling, or any other suitable means) are prevented from diffusingtoward the surface opposite the emitting surface (where the electronsmay recombine and cease to act as charge carriers) by the presence ofthe P layer 21, which as previously mentioned is doped with a highconcentration of a shallow acceptor such as beryllium. Other shallowacceptors, such as zinc or cadmium, may also be employed, The ionizationenergy of the beryllium acceptor impurity is less than 0.020 electronvolt, so that the beryllium acceptors are virtually completely ionizedat room temperature (kT being 0.026 electron volt at room temperature).The highly doped P layer 21 therefore pins the Fermi level near the topof the valence band, as shown in FIG. 3b. The resultant potentialbarrier 22 prevents electrons from reaching the P layer 21.

Instead of employing a highly doped P type layer to provide therequisite potential barrier, a suitable high work function metal(substituting for the layer 3) may be disposed adjacent the surface ofthe gallium phosphide layer 4 which is opposite the emitting surface, soas to provide a Schottky barrier which likewise prevents electrons fromdiffusing away from the emitting surface. A suitable metal for provisionof such a Schottky barrier is platinum.

In order for the dynode l to exhibit a negative electron affinity, it isnecessary that the bottom of the conduction band within thesemiconductor layer 4, at the point where sharp band bending commences,be

disposed at an energylevel above the vacuum energy level. In order tomeet this condition, the ionization energy E, of the deep acceptorimpurity should not exceed the difference between the energy gap valueE, and the work function I. Since it has already been stated that theionization energy E, should preferably be at least equal to a value onthe order of 4 kT, the deep acceptor impurity, the semiconductormaterial, and the electropositive surface layer should be chosen suchthat Preferably, the parameters of the dynode 1 should be chosen so thatthe distance d from the emitting surface at which the bottom of theconduction band crosses the vacuum energy level does not exceed a valuecor responding to a few times the mean free path for excited conductionband electrons within the semiconductor material.

When the structure described is employed, the distance A from theemitting surface within which sharp band bending takes place is lessthan a value on the order of a few times the mean free path for excitedelectrons, so that electrons in the conduction band may diffuse to theelectropositive layer 5 with a residual energy above the vacuum energylevel, and be emitted from the surface 6; no additional energy need besupplied to enable these electrons to surmount the potential barrier atthe emitting surface. The resultant effective electron affinity,corresponding to the difference between the vacuum energy level and theheight of the conduction band bottom above the Fermi level, is thereforenegative, as indicated in FIG. 3b.

Iron, the preferred deep acceptor impurity, has a room temperatureionization energy on the order of 0.8 electron volt. The work function Iof cesium on the P type gallium phosphide layer 4 is approximately 1.3electron volts, while the energy gap E, for gallium phosphide is 2.3electron volts. The ionization energy for the iron acceptors istherefore substantially greater than 4 kT (0.1 electron volt) and lessthan the difference between E, and I (1.0 electron volt).

With the aforementioned parameters, a beryllium impurity concentrationon the order of l0"/cm. in the p layer 21, and a thickness of 0.5 micronfor the layer 4, the energy bands are sloped so that a potentialdifference of approximately 0.2 volt exists across the portion of thelayer 4 denoted by the dimension e in FIG. 3a. The resultant electricfield within the bulk of the layer 4 has a relatively high value on theorder of 6 X 10 volt/cm., since the bulk of the P type layer 4 (i.e.-the portion where the energy bands are. essentially straight lines) isessentially insulating, having only on the order of 10 charge carriersper cm."'.

Although we have shown cesium as the material comprising theelectropositive work function reducing layer 5, a cesium-oxygencomposite layer may also be employed. Such a cesium-oxygen layer isknown in the art as a work function reducing material and is described,e.g., in the following reference:

A. A. Turnbull and G. B. Evans, Photoemission from GaAs-Cs-O, BRITISHJOURNAL OF APPLIED PHYSICS, Series 2, Volume I, Pages -160, Feb. 1968.

A partial cross-sectional view of the active portion of the injectionemitter shown in FIG. 2 appears in FIG. 4a, with a corresponding energyband diagram shown in FIG. 4b, FIGS. 4a and 4b being vertically aligned.

As in the structure of the dynode l, the deep acceptor (iron) doped Ptype layer 15 interacts with the thin electropositive work functionreducing cesium layer 18 to provide a negative effective electronaffinity at the exposed'surface of the cesium layer, and to establish apotential gradient (drift field) within the bulk of the layer 15 whichimpels electrons toward the emitting surface.

Electrons are injected from the heavily doped N type region 11 into theP type layer 15 across the forward biased P-N junction 16. Since ashallow donor such as 'tellurium is employed as the doping material forthe N region 11, the donor impurities are essentially completely ionizedat room temperature.

The deep acceptors in the portion of the P layer 15 adjacent the P-Njunction 16, however, are only very slightly (l0 ionized acceptors percm) ionized at room temperature. As a result, current flow across theforward biased P-N junction 16 is carried mostly by electrons (asopposed to holes), so that the P-N junction 16 serves as an efficientinjector of conduction band electrons into the layer 15.

Electrons so injected into the layer 15 are impelled toward the emittingsurface by the internal electric field indicated by the sloping energybands, so that high efficiency of electron emission from the exposedsurface of the cesium layer 18 is realized.

The various symbols in FIG. 4b correspond to similar symbols in FIG. 3b.

The dynode l, as shown in FIG. 1, may be fabricated by first evaporatingthe beryllium layer 3 onto the alumina substrate 2. The opticaltransmission characteristics of the beryllium layer 3 may be monitoredwhile the evaporation progresses, so that the evaporation may beterminated when an optical transmissibility corresponding to the desiredlayer thickness is observed.

The iron doped gallium phosphide semiconductor layer 4, which in thisexample is: polycrystalline (a monocrystalline layer may be employed),is deposited onto the beryllium layer 3 by, e.g., vapor phase reactionof gallium chloride (Gal,)|, ferrous chloride (FeCI and phosphine (H5),at a temperature on the order of 800 C. The deposition is continued forseveral minutes to deposit the polycrystalline gallium phosphide layer 4to the desired thickness. Under some conditions, the gallium phosphidelayer 4 may be essentially monocrystalline, even though grown onto thecrystallographically incompatible beryllium layer 3.

, During the growth of the gallium phosphide layer 4, beryllium diffusesfrom the layer 3 a short distance (10 to 100 Angstroms) into the galliumphosphide layer 4 to form the P* region 21 (see FIG. 3a).

The iron concentration in the gallium phosphide layer 4 may typically beon the order of 10" to 10 atoms/cm, as previously mentioned. However,high iron or other deep acceptor impurity concentrations are desirable,provided that they do not introduce excessive crystallographic strainsinto the gallium phosphide layer 4.

After cleaning the exposed surface of the gallium phosphide layer 4, athin layer 5 of cesium or cesiumoxygen is evaporated onto the galliumphosphide surface. The thickness of the layer 5 is monitored during theevaporation process by observing the photoemission current from thelayer 5 resulting from the illumination of the layer with light. Theevaporation of the layer 5 is terminated when the photoemission currentreaches a peak value. The resulting layer thickness, as previouslymentioned, is typically on the order of l to Angstroms.

Where it is desired to employ the dynode 1 as a transmission typesecondary emission structure, the alumina substrate may have a thicknesson the order of 500 Angstroms, so that primary electrons incident on theexposed substrate surface may penetrate to the gallium phosphide layer5. Techniques for manufacturing selfsupporting alumina'layers of thistype are well known in the art, and generally involve anodizing analuminum film to provide an alumina layer of the desired thickness, andsubsequently dissolving the aluminum film in a liquid, so that thealumina layer remains floating on the liquid surface.

The injection type electron emitter 10 shown in FIG. 2 may bemanufactured by providing a monocrystalline N type gallium phosphidesubstrate 11, masking one surface of the substrate with an insulatinglayer 13 comprising a material such as, e.g., silicon dioxide anddepositing a deep acceptor doped gallium phosphide layer 15 from thevapor phase onto the masked surface, so that the layer 15 growsepitaxially on the exposed gallium phosphide substrate surface and formsthe P-N junction 16 at the interface between the layer 15 and thesubstrate 11. The electrodes 12 and 17 may be evaporated ,onto thesubstrate 10 and layer 15, respectively, a suitable mask being employedfor evaporation of the electrode layer 17.

The electropositive cesium or cesium-oxygen layer 18 may then beevaporated onto the exposed surface of the P type gallium phosphidelayer 15, in the manner previously described. The battery 19 andresistor 20 are connected to the electrodes 12 and 17 by wires solderedor otherwise bonded thereto.

I claim:

l. A negative effective electron affinity electron emitter having aninternal drift field, comprising:

a P type semiconductor body;

a deep acceptor impurity at least in a region of said body att'lijacenta given electron emitter surface of said bo said impurity being presentin a substantially uniform concentration of atleast 10 atoms per cubiccentimeter,

a thin layer of work function reducing material coated on said electronemitter surface, said layer having a thickness on the order of from oneto ten Angstroms, and

means for exciting electrons into the conduction band of said P typesemiconductor body,

said semiconductor body, said acceptor impurity, and said work functionreducing material being chosen so that the ionization energy of saidacceptor impurity in said semiconductor body is greater than one-tenthelectron volt, but does not exceed the difference between the energy gapof the semiconductor body and the work function of the work functionreducing material.

2. The emitter defined in claim 1 wherein said work function reducingmaterial comprises a material selected from the group consisting ofcesium and oxygen.

3. The emitter defined in claim 1 wherein said semiconductor is galliumphosphide.

4. The emitter defined in claim 3 wherein said acceptor impuritycomprises iron and said concentration is on the order of 10 to 10 atomsper cubic centimeter.

5. The emitter defined in claim 1 comprising a potential barrier regionadjacent a surface of said body opposite said emitter surface, forpreventing diffusion of electrons away from said emitter surface towardsaid opposite surface.

6. The emitter defined in claim 1 wherein said barrier region comprisesa P region having a thickness substantially less than the thickness ofsaid semiconductor body between said emitter surface and said oppositesurface.

t i i I

2. The emitter defined in claim 1 wherein said work function reducingmaterial comprises a material selected from the group consisting ofcesium and oxygen.
 3. The emitter defined in claim 1 wherein saidsemiconductor is gallium phosphide.
 4. The emitter defined in claim 3wherein said acceptor impurity comprises iron and said concentration ison the order of 1017 to 1018 atoms per cubic centimeter.
 5. The emitterdefined in claim 1 comprising a potential barrier region adjacent asurface of said body opposite said emitter surface, for preventingdiffusion of electrons away from said emitter surface toward saidopposite surface.
 6. The emitter defined in claim 1 wherein said barrierregion comprises a P region having a thickness substantially less thanthe thickness of said semiconductor body between said emitter surfaceand said opposite surface.